The invention relates to a calibration method especially for calibrating a reflection spectrometric gauge used for measuring the quantity of a substance, such as oil, found as a thin film-like layer on a metal surface. Calibration is conducted by measuring the gauge response by means of a calibration unit and a solid, stable permanent sample provided on it as a calibration sample; and a change that simulates a certain substance concentration, e.g. oil concentration, is generated by the stable permanent sample in the reflection spectrum and utilized in calibrating the gauge response electronically, by software, or in some other way.
The invention also relates to a calibration unit especially for a reflection spectrometric gauge, comprising a support with one or several sections, a reflecting surface provided on the support, and a solid, translucent permanent sample provided on the reflecting surface.
The present invention relates to calibration of gauges operating on a reflection measurement principle, especially to calibration of IR analyzers, such as oil film gauges. An oil film gauge can be used for measuring the thickness of an oil film on a metal surface, such as a steel plate. With the gauge, the thickness of an oil film is measured by bringing the optical detector of the gauge to the vicinity of the surface to be measured, whereby the optics of the detector measure the reflection spectrum of the object within a certain wavelength range. The operation of an oil film gauge is based on partial absorption of the radiation emitted by the gauge into the surface measured. In gauges based on absorption spectroscopy, e.g. in oil film gauges, measurement is conducted by determining changes in the absorption of the radiation reflected from a substance in relation to the wavelength. The measurement is based on reflection spectrometry, in which IR radiation emitted from the light source of the gauge passes through an oil film found e.g. on a steel plate and measures the frequency of the functional groups, such as CH.sub.2 and CH.sub.3 groups, of the oil film in the sample by being reflected from the reflecting surface that underlies the oil film, such as the surface of the steel plate under examination, to the detector. On the basis of the reflection spectrum, the thickness of the oil film on the surface to be measured can be determined.
The reliability and accuracy of the measurement depend on the accuracy by which a spectral change caused by a measuring parameter, such as the thickness of an oil film, can be measured. The most significant factors that cause inaccuracy include changes in the intensity and radiation spectrum of the radiation source of the gauge, instability of the optical components used for separating wavelength ranges to be measured, changes in the geometry of the optics, drifting of the radiation detector components, and any changes occurring on the optical path of radiation, such as dirtying of the optics of the gauge.
To compensate for the errors caused by the above sources of error, the response, i.e. sensitivity, of the gauge has to be calibrated. Calibration is necessary to correct both the drifting of the signal levels of the gauge itself and the differences between different gauges. The purpose of the calibration is to make sure that the slope and zero point of the response graph are correct. If linearity is supposed, the response graph is a straight line.
In some known solutions for calibrating oil film gauges, an even oil film of a predetermined thickness is provided on a calibration unit, such as a sheet metal unit, and the response parameters of the gauge are adjusted by software in such a way that the gauge indicates the thickness of a known sample. However, an oil sample is unstable, and slow and difficult to produce.
Previously known are also such solutions according to the above introduction where a solid, stable, artificial permanent sample is provided on a calibration unit. The use of a permanent sample facilitates measurement and enhances reliability. This kind of solution is presented in European Patent 0,098,075. In the solution concerned, however, it is not possible to take into account all the factors that produce errors in calibration, especially not differences in the roughness of the objects to be measured. In the above-mentioned solution, the response line cannot be determined in a quick and simple manner, since there are no separate samples there that could be measured separately.